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Wednesday’s Rout Was An 8-Sigma Event: The 5th Largest Tail Event In History

With markets in rebound mode today, the sellside’s fascination with Wednesday’s sharp, unexpected selloff continues.

In the latest “hot take” on Wednesday’s dramatic drop, Goldman’s derivatives strategist Rocky Fishman takes on a different approach to the Wednesday rout, looking at it in terms of pre-event realized vol (of 6.4%), and notes that in this context, “Wednesday’s 3.3% SPX selloff naively represents an 8-standard deviation event, the 5th-largest tail event in the index’s 90-year history, as 6.4% annualized vol implies a 40bp one-standard deviation trading day; instead the drop was more than 330 bps.

As Fishman adds, what makes the drop unique is that most of the top events of this severity, and listed in the chart above, “have often had a clear, dramatic, catalyst (1987 crash, Eisenhower heart attack, Korean war, large M&A event breakup).”

Part of the reason this week’s volatility looks like a tail event is that realized volatility had been surprisingly low prior to Wednesday: the five least-volatile quarters for the SPX over the past 20 years were Q1/2/3/4 of 2017, and Q3 of 2018.

For Goldman, the Wednesday spike is reminiscent of the Feb. 27, 2007’s China-led selloff, “which marked the end of an extended low-vol period.”

That said, Fishman also notes that mathematical tail events have been more common recently, almost as if central bank tinkering with markets has broken them, To wit, “five of the top 20 one-day highest-standard deviation moves (comparing the SPX selloff with ex-ante realized vol) since 1929 have happened in 2016-8.”

Fishman then shift focus to the VIX, which while not as violent as the February record spike, “was also the 25th-largest one-day VIX spike on record.”

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